Nitrogen oxide emissions, principally nitrogen dioxide (NO.sub.2) and nitric oxide (NO), collectively referred to as NOx, have been linked to urban smog, acid rain and numerous harmful health effects. Of all NOx emissions produced in the United States, an estimated 55 percent is attributed to stationary sources such as utility boilers, industrial boilers, gas turbines and stationary engines.
The U.S. Environmental Protection Agency has promulgated New Source Performance Standards (NSPS) to define limits on allowable NOx emissions permitted from such stationary sources in order to abate the harmful effects caused by such emissions. However, NOx emission levels exceeding NSPS occur in many combustion facilities rendering the point source susceptible to fine and/or interruption of business.
In order to enhance the air quality surrounding such point sources and to promote cleaner burning combustion processes, three major approaches have been undertaken to reduce NOx emissions: (1) Making modifications before combustion; (2) Making modifications during combustion; and (3) Adding controls after combustion. Typical precombustion modifications include switching fuel stocks, emulsifying the fuel with water, and denitrifying the fuel. Typical combustion modifications include changing reaction stoichiometry, reducing combustion temperature and reducing process residence time. Adding controls after combustion is generally referred to as flue-gas treatment.
NOx reduction during combustion has been employed since the early 1970's and has achieved a limited degree of success in reducing NOx emissions. However, flue-gas treatment is typically required to obtain higher levels of NOx reduction and to meet the increasingly stringent NSPS. Flue-gas treatment consists of dry processes and wet processes. Dry flue-gas treatment processes are typically preferred over wet processes because they typically require less equipment and produce less waste that requires disposal.
Selective Catalyst Reduction (SCR) of nitrogen oxides using ammonia is currently considered one of the most efficient processes for removing NOx from flue gases. The SCR process is typically carried out on a titania supported vanadia catalyst employing NH.sub.3 /NO ratios near 1.0 and temperatures ranging from 300.degree. to 400.degree. C. to achieve conversions of up to 90%. NOx conversion increases with NH.sub.3 /NO ratio but higher ratios typically result in ammonia slip (ammonia breakthrough) which causes a secondary environmental problem.
The reaction pathway of SCR processes employing ammonia involves the oxidation of SO.sub.2 to SO.sub.3 by the vanadia catalyst followed by formation of NH.sub.4 HSO.sub.4 and (NH.sub.4).sub.2 S.sub.2 O.sub.7 which can cause corrosion and plugging of reactor components and catalyst deactivation. These problems coupled with equipment and operating costs associated with the storage, delivery and use of ammonia in SCR processes have led to a search for improved processes which do not utilize ammonia. However, such improved processes for removing NOx from oxygen-containing flue gases have eluded researchers.
Researchers have been investigating the use of hydrocarbons in the place of ammonia in SCR processes. Adlhart, et. al., R. E. Chem. Eng. Pro. 76, 73 (1971) studied the catalytic reduction of NOx in nitric acid tail gas using natural gas over alumina-supported platinum, palladium and rhodium catalysts. Results demonstrated that methane was the most difficult fuel to ignite among the fuels studied, requiring preheat temperatures of 480.degree. to 510.degree. C. Moreover, additional fuel in excess of the stoichiometric equivalent of total oxygen was required to completely remove NOx from the tail gas. For example, 1.7% methane was required to remove 0.23% NOx in tail gas having 3.0% oxygen at temperatures higher than 500.degree. C.
Limitations associated with the use of methane in processes for removing NOx from flue gas were confirmed in subsequent studies. Ault and Ayen, R. J., AlChE J. 17, 265 (1977), investigated the catalytic reduction of NOx in a substantially oxygen-free combustion stream. NOx-containing flue gas was reacted in the presence of hydrocarbons including methane, ethane, ethylene, acetylene, propane, propylene, octane, benzene and cyclohexane over a barium-promoted copper chromite catalyst in an oxygen-free atmosphere. Under reaction temperatures ranging from 225.degree. to 525.degree. C., an increase in the number of carbon atoms comprising the hydrocarbon reducing agent generally resulted in a decrease in the temperature required to effect the subject nitric oxide reduction. For example, about 10% NO was converted to the corresponding reduction products using methane as the reducing agent at 500.degree. C. wherein the nitric oxide inlet concentration was 1.0% and an amount of hydrocarbon 10% in excess of the stoichiometric requirement was employed.
Hamada and coworkers, Appl. Catal. 64, Ll (1990), Catal. Lett. 6, 239 (1990) studied the catalytic reduction of NOx in oxygen-containing flue gas using H-form zeolite and alumina catalysts and small amounts of propane and propene as reductants. The most active catalyst of three H-form zeolites studied was H-mordenite which gave the maximum nitric oxide conversion of 65% at 673.degree. K. followed by H-ZSM-5 and HY. Na-ZSM-5 provided a nitric oxide conversion of 32% at 573.degree. K.
The above-mentioned results suggest that NOx reduction efficiency depends not only on process operating temperatures but also on the type of catalyst and hydrocarbon employed as well as the amount of oxygen present in the NOx-containing flue gas. These factors have greatly impaired the ability to predict optimum catalysts and operating conditions in processes for removing NOx in combustion products such as flue gas.
Japanese Patent Application No. 291258/1987 discloses a zeolite catalyst for destroying NOx in automotive exhaust gas. The zeolite is ionically exchanged with a transition metal and carried on a refractory support. Preferred transition metals are copper, cobalt, chromium, nickel, iron and manganese. Copper is the most preferred. Preferred zeolites have a pore size ranging from 5 to 10 angstroms which is slightly larger than the molecular diameter of NOx. Methane was not disclosed as a reducing agent.
Iwamoto and coworkers (Shokubai 32, 6 430 (1990) demonstrated the effectiveness of NO reduction over copper-exchanged zeolite catalysts using H.sub.2, CO.sub.2, C.sub.2 H.sub.4, C.sub.3 H.sub.6 and C.sub.3 H.sub.8 as reductants. However, no data was presented for methane as a reductant. The rate of conversion to N.sub.2 increased with increasing O.sub.2 concentration and maximum conversion was obtained when O.sub.2 was between 0.8 and 2.0%. The authors concluded that the presence of oxygen was indispensable to the progress of the reaction but that a large excess of oxygen resulted in a decline in the NO removal rate. Further, during a public meeting, Iwamoto stated that methane was not an effective reducing agent for converting NOx in the presence of oxygen when Cu-ZSM-5 served as a catalyst.
U.S. Pat. No. 5,017,538 discloses an improved method for producing an exhaust gas purification catalyst which comprises a ZSM-5 catalyst carried on a refractory support. The ZSM-5 catalyst is ion-exchanged with a copper carboxylate and ammonia solution. Although no details are known about the reaction mechanism, preliminary studies suggest that catalyst activity and reaction selectivity vary with the hydrocarbon utilized with C.sub.3 H.sub.6 being preferable to C.sub.3 H.sub.8. The method is improved by a small amount of oxygen.
Operators of natural gas (methane) fired power stations, industrial boilers and combustion processes have been searching for an efficient and inexpensive catalytic reduction process for removing NOx from oxygen-containing flue gases. However, a catalytic process for destroying NOx in oxygen-rich combustion products utilizing methane as a reductant has not been reported.